1. Field of the Invention
This invention relates to an optoelectronic coupler with two photodiodes and an optic light guide, with one of the two photodiodes serving as the transmitting diode and the other as the receiving diode, while the light guide carries the light emitted by the transmitting diode to the receiving diode.
Optoelectronic couplers as described above are frequently used for digital data transmission, especially in cases where, as explained below, explosion-protection regulations require the electroconductive separation of any two electrical circuits.
In various fields of technology, as for instance in flow-rate and fill-level measurements, it is often necessary to protect the equipment employed against explosion hazards. To obtain such explosion protection, functional areas requiring different treatment must be kept separate from one another. Such separation is imperative especially for measuring systems that receive their electric energy via separate AC lines and are usually referred to as four-wire units. As differentiated from these four-wire systems, two-wire systems are those in which the electric power supply and the data transmission share one common line, obviating the need for a separate electric line.
2. Description of the Prior Art
The aforementioned four-wire systems most commonly receive their electric power with “enhanced safety” type of protection. Such “enhanced safety” type of protection involves provisions whereby a higher level of safety precautions prevents inadmissibly high temperatures as well as sparking or arcing on the inside or outside of electrical components where such phenomena are not supposed to occur in normal operation. The type of protection provided on the side of the sensor which in the case of a magnetoinductive flowmeter would be on the side of the electrodes, or in the case of a Coriolis-type mass flowmeter on the side of the driver and the sensor coils, is usually in the form of “inherent safety”. For the purpose of “inherent safety” the energy in the circuit is maintained at a level so low that no sparks, arcs or temperatures capable of causing an ignition can be generated. With “inherent safety”, even short-circuiting will not produce a spark that could cause an ignition leading to an explosion.
One way to obtain “inherent safety” involves so-called “barrier technology”, which essentially employs limiting circuitry toward “safe ground”. For measuring purposes, however, such relationship with “safe ground” is susceptible to interference due to ground return loops while also being error-prone because the connection to “safe ground” must generally be provided by the user of the measuring instrument concerned when that is put in operation. It has, therefore, been found safer and therefore desirable to employ conductively separated circuits, obviating the need for “safe ground” limiting provisions.
In the case of four-wire devices, the conductively separated circuits must carry electric power and, respectively, handle typically digital data communication. The electric power is generally supplied via DC/DC converters with safely separating isolation transformers, while digital data communication usually takes place via optoelectronic couplers.
For these optoelectronic couplers, handling the digital data transmission, to be adequately explosion-protected, minimum gap requirements have been established for the so-called air and creepage paths. For example, the distance between the electrically separated circuits must correspond to a 10 mm creepage path or consist of 1 mm solid insulation whenever the maximum allowable voltage is 375 V, i.e. when the power supplied is 230V line voltage. In that case, only certain, specially approved optoelectronic couplers may be used, featuring a minimum of 1 mm solid insulation between the transmitting diode and the receiving diode, provided the insulating layer is guaranteed to withstand the maximum temperature permissible from the safety point of view and provided the aforementioned 10 mm creepage gap on the circuit board carrying the diodes is maintained. This creepage gap of at least 10 mm on the circuit board applies in particular to the pin spacing of the optoelectronic couplers and to the pad spacing on the circuit board.
Yet optoelectronic couplers that meet the aforementioned requirements are relatively expensive, they are predominantly in the form of hard-wired components that involve complex installation, they tend to be bulky because of the necessary spacings referred to above and the additional protective provisions, and given the minimum of 1 mm insulating space, their optical coupling is poor. This is why optoelectronic couplers of this type most often fail to achieve rapid data transmission.